Satgen 378    The Picosat Concept      by GM4IHJ          22nd June 96

   As mentioned in Satgen377, students at Stanford University in
California plan to construct a fleet of 16+ small satellites, each of
which will have the capability to transpond 38+ ACSSB or 9+ FM signals
, from low earth orbit. This exciting new concept is not without its
problems. How will they be overcome ?

THE FM SIGNAL. As both Amsat 27 a single channel FM transponder , and
Mir/Shuttle SAREX FM voice qsos have demonstrated, you either restrict
access to specific stations such as schools or clubs, or , the high
powered alligators take over the channel. A situation consequent on
the FM Capture principle, whereby when several FM stations all uplink
to the satellite, only the strongest signal is heard/transponded.

On mode J Ao27 the strongest signal over Europe is an intruding
Spanish taxi controller, who denies the channel to anyone else by
using high power on the 2m uplink with no concern for the fact that it
is a satellite frequency. Picosats are planned to use mode L , which
will certainly reduce this problem. However the FM capture problem
will remain and it is not clear how it can be dealt with.

THE ACSSB SIGNAL. According to the Picosat brief Amplitude Compandored
Single side band will be used. Whether "Compandored" is a typo is
uncertain, the word is not in any standard dictionary known to this
writer. But the further description reveals it to be a variant of the
partially suppressed carrier mode of SSB which has been around since
the late 1940s, albeit with Picosat having a variation where the pilot
carrier is displaced 3.1 Khz from the original carrier. This is a
proven method for applying Auto Frequency control techniques which
will certainly be necessary in view of Picosats expected 70 Khz range
of doppler shift , consequent on its non inverting transponder.
Presumably any call for an inverting transponder which would reduce
doppler range to about 40 Khz maximum, is negated because it
complicates the Tx and Rx circuitry by causing a switch of the
sidebands between transmit and receive from say LSB to USB . Despite 
70Khz doppler the method used by G3RUH Pacsat modems for AFC tracking
of PSK should be applicable to Picosat ACSSB.

CHANNELISATION . It is proposed to channelise both the FM and ACSSB
modes of the Picosats. Clearly this will be desirable but difficult to
implement . Noting that a station about to leave the satellite has a
very different notion of Ch 1, from that of a station just joining the
sat, who wants to talk to him. Equally difficult will be the problem
of switching satellites as stations leave one footprint then enter
another whilst stations are in qso. The presence of a CW beacon on
each Picosat may be a useful doppler frequency marker in these
situations. But it is perhaps timely to ask if Stanford could release
circuitry details for the ACSSB, so that would be users can get in
some ACSSB practice on existing mode A and J amsats.

So, we must go up to mode L with lower bands becoming untenable, and
here is a superb proposal from Stanford to start us moving. A proposal
which combines real innovation and something for all Radio Amateurs.
AMSAT UK Colloquium 96 attendees please note .